Add mac clang toolchains required for V8 cross compiling

build/toolchain/mac/BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # TODO(brettw) Use "gcc_toolchain.gni" like the Linux toolchains. This requires
 # some enhancements since the commands on Mac are slightly different than on
 # Linux.
 
 import("../goma.gni")
 import("//build/config/clang/clang.gni")
@@ skipping 413 matching lines @@
   }
 }
 
 mac_toolchain("clang_x64") {
   toolchain_args = {
     current_cpu = "x64"
     current_os = "mac"
   }
 }
 
+mac_toolchain("clang_x86") {
+  toolchain_args = {
+    current_cpu = "x86"
+    current_os = "mac"
+  }
+}
+
+mac_toolchain("clang_x86_v8_arm") {
+  toolchain_args = {
+    current_cpu = "x86"
+    v8_current_cpu = "arm"
+    current_os = "mac"
+  }
+}
+
+mac_toolchain("clang_x86_v8_mipsel") {
+  toolchain_args = {
+    current_cpu = "x86"
+    v8_current_cpu = "mipsel"
+    current_os = "mac"
+  }
+}
+
+mac_toolchain("clang_x64_v8_arm64") {
+  toolchain_args = {
+    current_cpu = "x64"
+    v8_current_cpu = "arm64"
+    current_os = "mac"
+  }
+}
+
+mac_toolchain("clang_x64_v8_mips64el") {
+  toolchain_args = {
+    current_cpu = "x64"
+    v8_current_cpu = "mips64el"
+    current_os = "mac"
+  }
+}
+
 if (is_ios) {
   mac_toolchain("ios_clang_arm") {
     toolchain_args = {
       current_cpu = "arm"
       current_os = "ios"
     }
   }
 
   mac_toolchain("ios_clang_arm64") {
     toolchain_args = {
       current_cpu = "arm64"
       current_os = "ios"
     }
   }
 
   mac_toolchain("ios_clang_x86") {
     toolchain_args = {
       current_cpu = "x86"
       current_os = "ios"
     }
   }
 
   mac_toolchain("ios_clang_x64") {
     toolchain_args = {
       current_cpu = "x64"
       current_os = "ios"
     }
   }
 }